Process of etching aluminum foil for electrolytic capacitor



PROCESS. OF ETCHING ALUMINUM FOIL. FOR ELECTROLYTIC CAPACITOR Filed April 6. 1956 p 23, 1958 A J. CATOTTI ET AL I 2,853,445

ATTORNEYS PROCESS OF ETCHING ALUMINUM FOIL FOR ELECTROLYTIC CAPACITOR Arthur J. Catotti, Marion, Hans Cohn, New Bedford, and Peter P. Grad, South Dartmouth, Mass., assignors it]: Aerovox Corporation, a corporation of Massausetts Application April 6, 1956, Serial No. 576,673

8 Claims. (Cl. 204-441) United States Patent to produce a microscopically roughened surface presenting a substantially greater area than that of the smoothfoil. Subsequent to etching, the foil undergoes an anodizing treatment whereby a film of aluminum oxide is formed on the foil. Greater surface areas of aluminum will naturally produce greater areas of interfacial contact between the aluminum and the oxide film, which, in turn, is desirable in that the capacitance of a condenser made'in this way is directly proportional to this area of interfacial. contact. Such etching processes have been carried out both chemically and electrochemically with the chemical method being more widely used because of the smaller initial investment required to install the equipment. The usual chemical etching process is carried out by immersing the aluminum foil in a dilute aqueous solution of a highly ionized mineral acid, such as hydrochloric acid, for a predetermined period. The process has been refined in various ways, such as by the addition of various catalytic agents, such as aluminum chloride, to the etching bath. Chemical etching processes, however, have proved difficult to control in normal manufacturing operations. It has been found that a very minute deviation in the amount of impurities contained in the aluminum foil to be etched ,causes very substantial difierences in the amount of etching obtained and in the rate of etching. For example, a'chemical etching procedure that is satisfactory for aluminum foil of 99.85% purity, would, in many cases, be totally useless for a foil of 99.80% purity. Thedifference of 0.05% in the amount of impurities in such a case often accelerates the etching process to such an extent that the etched solution burns holes in the foil or produces a surface coating of loose scales. On the other hand, the use of a slightly 'purer foil, for example, one having an aluminum content of 99.90%, would result in such a' sharp decrease in the rate of etching'that an undesirably small gain in surface area would result fromthe etching process. I 1

.From the above it will be apparent that to carry out a conventional etching process extremely close and accurate control must be maintained in purity of the aluminum'foil used; Suchcontrol is not always possible due to 'variationsin large .scale production of foil. For this reason, substantial amounts of aluminum foil are oftenlost to. the capacitor manufacturer-due. to burning Patented Sept. 23, 1958 or scaling, as above pointed out, and this may materially offset the cost advantage such a process has over electrochemical etching methods.

Because of the increased rate of etching that has been found to accompany the presence of greater amounts of impurities in aluminum foil, it is generally believed that some of these impurities act to catalyze the etching process. This theory is further borne out by the fact that conventional chemical etching processes are relatively ineffective in the etching of high purity aluminum foil, that is, foil containing at least 99.99% aluminum. Etching of such high purity aluminum foil can be carried out only by chemical methods that maintain the temperature of the electrolyte at an undesirably high level, such as'in the. order of C., or by the useof relatively ex- Another object of the invention is to provide a chemical I etching process for use with high purityaluminum foil which can operate at relatively low and easily controllable temperatures.

These and other objects of the invention, which will be in part specifically pointed out and in part apparent from the subsequent description, are attained by a gal.- vanic etching process in which an electrical conductor is connected between the aluminum foil to be etched and an electrode of an'element more noble or more electropositive than aluminum while maintaining the'foil and the electrode in contact with an electrolyte comprising an aqueous solution of an acid and a 'salt that has a cation which is not in its lowest state of oxidation and which is capable of forming a soluble salt in a 'lower oxidation state. The aluminum is oxidized to form a solution of aluminum salt of the acid while'the other salt is reduced to its lower oxidation stage. 'In such a process no insoluble reduced product is formed that must be disposed of and the solution can be renewed from time to time by addition of further amounts of reducible salt.

In a preferred embodiment of the invention, the elec ferred electrolyte is made up of an aqueous solution of hydrochloric acid and ferric chloride. Duringthe etching process,'the aluminum is oxidized to aluminum chloride which remains in solution in the electrolyte The use of ferric chloride is ofparticular advantage since this solution can be regenerated to ferrous chloride simply by blowing air through the spent solution. The carbon electrode, being inert, has no tendency to dissolve in the acid solution, as would a metal electrode, even though the metal were below aluminum (but above hydrogen) in the electromotive series. Thus, no reaction takes place tending to oppose the main reaction, i. e. I

Instead of an inert electrode, it is sometimes more convenient to employ a metal electrode composed-of an .element below aluminum in the. electromotive series. A preferred electrode .of this class is iron because it, is inexpensive and readily available. However, chromium, cobalt, lead, copper, silver, platinum and gold may also be employed as electrodes, provided they do not contaminate the particular etching solution employed by entering into side reactions with it. The more noble metals are more effective because of the greater electromotive force set up, but this advantage is more than offset by the high price of such materials. In addition to pure metals or elements, alloys may also be employed and the term element as used hereinafter is intended to include alloys.

The process of the present invention is illustrated by a single figure of drawings which show diagrammatically apparatus for carrying out the etching process. 7 Referring now specifically to the drawing, there is indicated generally at 10 a tank, containing an electrolyte 11, into which is fed the aluminum foil 12 to be treated, which foil passes through the tank under the submerged roller 13 and emerges over the roller 14. The speed with which the aluminum strip 12 is fedthrough the tank 10 is regulated so as to provide for a controlled period of immersion in the electrolyte 11. As the strip 12 enters the tank it comes into contact with an electrical connector, such as brush shown at 15, which is connected to a conducting wire 16. Also immersed in the electrolyte is the electrode 17 which is connected to the brush 15 by means of the wire 16. The electrode 17 is made of some element below aluminum on the electromotive series and is preferably made of carbon or desirably of metallic iron, although lead, copper or other preferably inexpensive ele ment could be used in place of iron, if desired.

The electrolyte 11, through which the aluminum foil 12 is passed, is made up of an aqueous solution of a strong mineral acid and a salt of a metal that has at least two oxidation stages, which salt is soluble in each of its oxidation stages. Among the metals whose salts possess this property are iron, copper, nickel, cobalt, chromium and tin among others, of which iron is generally preferred. While any soluble salt would be suitable for this purpose, such as the nitrates, sulphates, or the like, the chlorides are generally preferred. Thus, the chloride of iron is generally to be used.

The nature of the acid employed also admits of substantial variation. It should be understood that in the process of the present invention, the function of the acid is not to attack the aluminum dire-cly as in conventional etching. The aluminum is dissolved by a reaction with a reducible metallic ion, suchas the ferric ion. The acid merely serves to prevent precipitation of basic salts of the reducible metal. Thus, any acid may be employed, although mineral acids, such as hydrochloric acid, are preferred for economic reasons. Preferably, the acid and the reducible salt should have the same cation so that the etching process may be aided by the common ion efiect.

'The concentration of the acid and salt in the electrolyte is not believed to be particularly critical to the operation of the etching process. Since the acid does not function as a direct etchant, its concentration may be substantially lower than that used in conventional etching solutions. Thus, by employing only enough acid to prevent formation of insoluble salts, the desirable result is attained of minimizing the gaseous hydrogen formed by the direct reaction of aluminum with acid. A convenient concentration of hydrochloric acid has been found to be 1 N, although concentrations as low as .01 N and as high as 2 to 3 N may be employed.

Concentration of the ferric chloride is determined primarily by the amount of etching desired since three ferric ions are used for every aluminum ion formed. The concentrations of the acid and ferric chloride are preferably about the same order of magnitude, although there appears to be no critical relation between the concentration of these constituents. Thus, the ferric chloride concentration may vary between about .01 N and 3' N.

The temperature at which galvanic etching is carried out deter-mines its effectiveness as measured in terms of so-called capacitance yield. Capacitance yield is the number of square inches of foil necessary to produce one microfarad of capacitance after the foil has been formed with an aluminum oxide film. In order to effect economy, it is naturally desirable that this capacitance yield be as low as possible. It has been found that in the galvanic etching of high purity aluminum foil, it is possible to obtain substantial etching and satisfactory capacitance yields at much lower temperatures than can be employed using conventional chemical etching methods. At temperatures in the range of about 20 C. to about C. it has been found that capacitance yield improves as the temperature increases to a maximum in the neighborhood of about 80 C., the capacitance yield tending to become poorer with further temperature increase. In practice a substantially constant temperature of the etching bath of between about 30 C. and about 80 C. is preferred.

Table I, below, illustrates the variation of capacitance yield with etching temperature. Three different values are given for capacitance yields, i. e. the yields obtained by forming the oxide film at 100, 300 and 600 volts. The oxide film is formed at the different voltages depending upon the maximum voltage to which the capacitor is ultimately to be subjected. The data set forth in Table I, as Well as the subsequent tables, was obtained by the following etching procedure:

The foil is first washed for 30 seconds in a 2% aqueous solution of sodium hydroxide maintained at about 82 C. The sodium hydroxide is then removed by washing in distilled water and the foil is etched in the manner above set forth. Following etching, the foil is again washed in distilled water, followed by washing in dilute nitric acid maintained at about 65 C. for about 30 seconds. Following this acid treatment, the foil is again washed and then boiled in distilled water and then dried and subsequently oxidized and tested. In Table I, below, all of the samples were subjected to five minutes of galvanic etching in a solution of 1.09 N I-lCl and 0.135 N FeCl A soft iron anode was employed.

TABLE I Efiect of temperature on capacitance yield of 99.99% foil (F -f0i l) Capacitance Yield, Sq. Etch Percent In./Mfd. Sample Temp, Wt.

0. Loss 100 300 600 Volts Volts Volts The elfect of etching time on capacitance yield has been found to be somewhat similar to that of etching temperature in that there appears to be an optimum time. In Table II, below, there is illustrated the results of etching various grades of aluminum foil galvanically for varying periods. In Table II, three types of foil were used. The samples indicated as B foil were 99.80% aluminum. The C samples were 99.85% aluminum. The F samples were high purity aluminum foil containing 99.99% aluminum of the same types employed in Table I. A number of samples of foil were etched galvanically, using an iron anode in a solution of 1.05 N HCl and 0.185 N FeCl at 45 C. The results given in Table II, below, indicate that for both the B and C foil, improvement TABLE 11 Capacitance Yield, Sq. Etch Percent In./Mid. Sample Time,

Min. Loss 100 300 600 Volts Volts Volts 15.1 1. 34 3. 40 6. 64 2 8.85 62 2.10 4. 00 p 4 11.9 .392 1.28 2.70 6 13. 7 284 1. 13 2. 46 8 18. 5 274 784 2. 06 28. 8 .235 .89 1.82 12 33.1 233 .74 1.57 24. 2 215 784 1.74 0 l1. 3 2. 90 6. 3O 2 12. 7 535. 1. 73 3. 76 4 17. 5 .442 1.33 2. 90 6 21.0 .348 1.15 2. 43 8 24.0 .256 815 1. 87 10 32. 5 226 675 1. 63 10 22.8 .79 l. 70 10 28.4 .22 79 1. 67 v 12 30.6 .176 .70 1.60 14 31.8 .20 .69 1.62 0 10. 4 2. 48 5. 95 4 17.1 605 2.10 4. 6 19. 3 680 1. 85 4. 00 8 20.0 475 1. 56 3. 45 10 23. 8 .416 1.41 3.13 12 27. 4 435 1. 41 3.02 13 27.2 .416 1.18 2. 70 14 28. 1 .325 1.05 2. 42 17 32. 6 342 l. 10 2. 70 20 33. 6 290 1. 01 2. 24

One of the principal advantages of the present invention over more conventional chemical etching methods is that it etches aluminum foil to an extent that is less dependent on the amount of impurities present. This is shown by comparison of the results of Tables III and IV, below, with Table V, below. Tables III and IV show the results obtained by etching two types of aluminum foil in a conventional etching bath consisting of an aqueous solution of 2.8 N HCl and 0.52 N AlCl for 95 seconds at 60 C. and 65 C., respectively. Table V shows the results obtained by etching the same two types of aluminum foil galvanically with an iron anode, according to the present invention, in an aqueous etching solution hav ing a concentration of 0.78 N HCl and 0.185 N AlCl at a temperature of 45 C. for the times thus indicated on the table. In these tables the results are averages taken from numerous samples.

TABLE III Chemical etch at 60 C.

6 TABLE v Galvanic etch at 45 C.

Capacitance Yield, Etch Sq. In./Mid. Sample Time, Percent Min. Wt. Loss 100 300 600 Volts Volts Volts The importance of these tables isnot in a comparison of the capacitance yields obtained by the. galvanic etching as opposed to conventional chemical etching, but rather in the divergencies of values obtained for B foil and C foil etched by the two different processes. When etched by the conventional chemical method, there is a very substantial difierence between the capacitance yield obtained for B foil and that obtained for C foil, despite the identity of the etching conditions. For example, from Table III, it will be noted, that the capacitance yield obtained for C foil was at some voltages over 100% higher than that obtained for B foil. This means that a difference of impurities content of only 0.05% results in a tremendous difference in capacitance yield when conventional chemical etching is used. This means that a small deviation from the specification of the foil for which the process is adapted will result in tremendously different electrical properties of the formed film. On the other hand, Table V shows that when the etching process of the present invention is employed, the same difference of impurity content results in only a relatively minor divergence in capacitance yield. Thus, a small deviation from the specification of the foil will not have the wide divergence in capacitance of the resultant capacitor which is inherentwhere conventional chemical etching processes are used.

In all of the examples heretofore given, the galvanic etching of the present invention has been carried out employing an iron anode. However, it has been found that other elements beside iron are suitable for this purpose, provided that they are below aluminum in the electromotive series. This includes also those elements which are electrochemically inert, i. e. have no tendency to form ions, but which are sufiiciently good electrical conductors to complete the galvanic circuit. One such inert element that is particularly preferred is carbon. Carbon anodes yield particularly beneficial results when employed in conjunction with a low acidity etching solution. Even a metal below aluminum in the electromotive series (such as iron) has a tendency to react with the acid etching solution, thus setting up an electrochemical force opposite to the direction of the overall galvanic etching reaction, i. e.

Al+3Fe+++- Al++++3Fe++ The results obtained by the etching of high purity (99.99% pure) aluminum foil in conjunction with a carbon anode are set forth in Table VI, below. In this case the foil was immersed for various times in a solution of 0.110 HCl and 0.195 FeCl at a temperature of 45 C.

The results set forth in Table VI below may be contrasted with those obtained by etching the same high purity aluminum foil bya conventional method at temperatures of about 60 C. using as an etchent an aqueous solution 2.8 N HCl and 0.52 N A1Cl at etching which was seconds, so that these samples are comparable with those etched galvanically for 1 /2 minutes and recorded in Table VI below. A comparison of the results of Tables VI and VH clearly illustrates that the galvanic TABLE VI Capacitance Yield Av- Number Wt. Loss erage, Sq. In./Mfd Time in Min. of vg.,

Samples Percent 100 300 600 Volts Volts Volts etching of the present invention is much more effective than conventional etching, even when using substantially lower temperatures and more dilute etching solutions than those of Table VII.

Although the present invention has been described with respect to a number of specific embodiments thereof, it should be understood that it is not limited to such embodiments but embraces also those reasonable equivalents occurring to those skilled in the art.

What is claimed is:

1. The process of etching aluminum foil containing small amounts of impurities, wherein variation in the degree of etching obtained due to variation of the amount of impurities in said foil is reduced, comprising the steps of connecting an electrical conductor between said foil and an electrode spaced therefrom, said electrode comprising an element more noble than aluminum and insoluble in the etch solution, while maintaining said foil and said electrode in contact with said etch solution, said solution comprising an aqueous solution of a mineral acid and a salt of said acid, the cation of said salt being in the higher of two soluble oxidation states.

2. The process of etching aluminum foil for use in an electrolytic capacitor, said foil containing small amounts of impurities, wherein variation in the degree of etching obtained due to variation of the amount of said impurities is reduced, comprising introducing the foil into an aqueous solution of hydrochloric acid and ferric chloride, electrically connecting Said foil to an electrode exteriorly of said solution and insoluble in said solution and progressively passing said foil through said bath out of contact with said electrode to etch the surface of said foil.

3. The process according to claim 2 wherein said electrode comprises iron.

4. The process according to claim 2 wherein said electrode comprises carbon.

5. The process according to claim 2 wherein the concentration of said hydrochloric acid is from about 0.1 N to about 3 N and of said ferric chloride is about 0.1 N to about 3 N.

6. The process according to claim 2 wherein the temperature of said bath is maintained between about 20 C. and about 80 C. and said foil is maintained in contact with said electrolyte for not longer than about 12 minutes.

7. The process of etching aluminum foil containing at least 99.99% by weight of aluminum prior to anodizing said aluminum for use in an electrolytic capacitor, comprising the steps of immersing at least a part of a carbon electrode in a bath of an electrolyte comprising an aqueous solution of hydrochloric acid and ferric chloride, establishing electrical connection between said electrode and said foil exterior of said bath and progressively passing said foil through said bath out of contact with said electrode to etch the surface of said foil and subsequently washing said aluminum foil.

8. The process according to claim 7 wherein the temperature of said bath is maintained between about 20 C. and about 80 C.

References Cited in the file of this patent UNITED STATES PATENTS 2,052,962 Booe Sept. 1, 1936 2,162,789 Raub June 20, 1939 2,336,846 Clark Dec. 14, 1943 2,699,382 Altenpohl Ian. 11, 1955 FOREIGN PATENTS 854,921 France Ian. 29, 1940 

1. THE PROCESS OF ETCHING ALUMINUM FOIL CONTAINING SMALL AMOUNTS OF IMPURITIES, WHEREIN VARIATION IN THE DEGREE OF ETCHING OBTAINED DUE TO VARIATION OF THE AMOUNT OF IMPURITIES IN SAID FOIL IS REDUCED, COMPRISING THE STEPS OF CONNECTING AN ELECTRICAL CONDUCTOR BETWEEN SAID FOIL AND AN ELECTRODE SPACED THEREFROM, SAID ELECTRODE COMPRISING AN ELEMENT MORE NOBLE THAN ALUMINUM AND INSOLUBLE IN THE ETCH SOLUTION, WHILE MAINTAINING SAID FOIL AND SAID ELECTRODE IN CONTACT WITH SAID ETCH SOLUTION, SAID SOLUTION COMPRISING AN AQUEOUS SOLUTION OF A MINERAL ACID AND A SALT OF SAID ACID, THE CATION OF SAID SALT BEING IN THE HIGHER OF TWO SOLUBLE OXIDATION STATES. 